1. Technical Field
The present invention relates generally to ion implanter systems, and more particularly, to a method, system and program product for tuning an ion implanter system to maximize ion beam implant current by using an estimated implant current, and to position the ion beam along a desired ion beam path by using a spot beam center.
2. Related Art
Ion implantation processes typically require a uniform and consistent dose or amount of ions to be implanted into a semiconductor wafer. Dose is generally a function of ion beam current density and time that the wafer spends in front of an ion beam. Referring to FIG. 1, conventional single wafer ion implanters typically provide an ion beam that is horizontally an electro-statically scanned spot beam 4. In order to scan a wafer 6, one conventional approach moves the wafer vertically (as shown by the three wafer 6 positions in FIG. 1) and ion beam 4 is swept horizontally back-and-forth across wafer 6. During this process, in order to achieve a uniform dosage, ion beam 4 has to be swept completely off the edges of wafer 6, which results in an oversweep of width d on each side of wafer 6. Since oversweep size d is proportional to ion beam 4 width D in a sweeping direction, the beam width at the wafer plane has a significant effect on the amount of beam 4 that can be used, i.e., an implant current of the beam. In particular, oversweep creates a situation in which a fraction of the entire beam is available for use, which impacts ion implanter system productivity.
In order to address this situation it is advantageous to maximize ion beam implant current, which is determined by total ion beam current and ion beam spot width. “Implant current” is the quantity of current at which the ion beam impacts the wafer, and is to be distinguished from total ion beam current, which is merely the cumulative current of the ion beam available through one sweep. Implant current is a function of total ion beam current and ion beam spot width, and increases as total ion beam current increases and ion beam spot width decreases. On the other hand, ion beam spot width is a function of total ion beam current and increases as total ion beam current increases. Accordingly, total ion beam current and ion beam spot width have to be optimized simultaneously to maximize implant current. On conventional single wafer ion implanters, however, total ion beam current tuning and beam spot width profiling are accomplished separately. In particular, the total ion beam current is tuned at a set-up Faraday cup that may be moved into and out of the path of the ion beam. Ion beam spot width is measured at a wafer plane by a traveling Faraday cup profiler, as disclosed, for example, in U.S. Pat. No. 4,922,106. The beam transmission from the set-up Faraday cup to wafer is not controlled.
One approach that considers beam width relative to controlling an ion implanter is disclosed in U.S. Pat. No. 6,614,027 to Iwasawa. Iwasawa discloses a method and apparatus capable of preventing the total ion beam current of a swept charged particle beam from becoming smaller than a prescribed value when an electrostatic lens is adjusted by having the control consider the width of the charged particle beam in a sweeping direction. Iwasawa generates a unified evaluated value, which considers beam width and current, and then controls a lens focusing voltage Vf(n) to maximize this value. In particular, maximization of the unified evaluated value is achieved by selecting a lens focusing voltage Vf(n) that increases total ion beam current and minimizes a beam width deviation from an ideal beam width. That is, Iwasawa intends to achieve the prescribed large swept total ion beam current with as little deviation in beam width as possible. In so doing, Iwasawa approximates the total ion beam current and ion beam spot width to their preferable states, but does not otherwise attempt to achieve an optimized total ion beam current and optimized ion beam spot width to maximize implant current. More specifically, it is impossible for Iwasawa to maximize implant current because it is incapable of simultaneously optimizing total ion beam current and ion beam spot width to achieve this goal.
Iwasawa is also limited in the ability to affect these parameters because it controls only the one lens. That is, more than one beam optical component in an ion implanter system must be controlled to optimize total ion beam current and ion beam spot width simultaneously to maximize implant current. As a result, Iwasawa is incapable of adequately addressing the underutilization of an ion beam caused by oversweep.
Another challenge to optimizing ion implanter productivity is to minimize beam set up time. In conventional ion implanter systems, set up requires a number of measurements and subsequent tuning steps to be conducted, such as ion beam implant current tuning as described above, parallelism tuning and uniformity tuning. The challenge is increased because it is common to repeatedly use the same group of ion beams for implanting different wafers. As a result, the same ion beams need to be set up over and over again during continuous production. It is desirable to have constant beam geometry for each ion beam over many setups to provide consistent semiconductor device performance.
In view of the foregoing, there is a need in the art for a technique to maximize ion beam implant current as well as minimize ion beam set up time and otherwise improve ion implanter system productivity.